This invention relates generally to electrical terminations, and more particularly to termination networks used to terminate electric signal lines.
As it is known in the art, terminations are commonly used to absorb energy of electrical signals at the end of transmission lines to prevent this energy from being reflected back into other circuitry. In digital applications, unlike microwave applications, an ideal termination is a resistor having no significant inductive or capacitive components. One known layout provides termination resistors in an integrated circuit package with six to eight resistors each having one lead sharing a single common return conductor coupled to one pin of the package, which is used to provide the terminator reference voltage and with the other lead of each resistor coupled to corresponding pins of the package. Multiple terminating resistors are available from many vendors in single-inline packages (SIPs). These packages are compact, inexpensive, and offer an efficient way to terminate many signals in a small area, as from a common bus, for example.
A basic problem encountered in the physical design of the SIPs electrical termination schemes, is that, generally for high frequency signal termination with multiple resistors in a package, there is a relatively high intrinsic inductance from the power or reference pin to the individual resistors. Signals on the transmission lines that are being terminated place current through the resistors, and through the common inductance of the return lead. A voltage is generated across the intrinsic inductance of the lead by the changing signal current, according to V=-Ldi/dt. This induced voltage appears as noise on the signal paths.
Thus, the problem of high inductance is aggravated because there is noise caused by the rapidly changing current through the inductance of the power and reference paths to the resistors.
Further, the inductance is large because the common return path tends to be physically long. This path is also shared by a number of signals, such that the total current through the inductance is increased. Therefore, the noise generated is large, especially when the signal rise-times are fast and the frequencies are high.
In the prior art, this high inductance was not a obstacle with lower speed signals (ie. in the 5-10 nanosecond rise-time range). However, today, signal rise times have increased (ie. in the one nanosecond rise-time range and less), primarily due to the development of faster components.
One common solution to the intrinsically high inductance associated with high frequency signal termination with multiple resistors in a package to some degree is to use discrete chip surface mount resistors and resistor networks with integral bypass capacitors to compensate for the inductance. However, a problem with this solution arises because the chip resistors are all discrete parts and their use requires more parts placements on the boards taking up more "real estate" and potentially lowering yields.
Another solution for decreasing inductance is to mount a small capacitor on the resistors in the multiple resistor package. The capacitors serve to compensate for the loop inductance by providing current for the resistors locally, thus bypassing the inductive path for short term current frequencies. However, this method is more expensive than a regular resistor package (SIP) without integral capacitors and requires a multi-pass assembly process which is prone to some error. Further, the capacitance may not be the correct value for all frequency ranges of signals. The capacitors also introduce resonances into the terminator's characteristics which could cause unwanted signal distortion.